U.S. patent number 7,393,733 [Application Number 11/001,166] was granted by the patent office on 2008-07-01 for methods of forming hybrid fin field-effect transistor structures.
This patent grant is currently assigned to AmberWave Systems Corporation. Invention is credited to Matthew T. Currie.
United States Patent |
7,393,733 |
Currie |
July 1, 2008 |
**Please see images for:
( Certificate of Correction ) ** |
Methods of forming hybrid fin field-effect transistor
structures
Abstract
Semiconductor-on-insulator structures facilitate the fabrication
of devices, including MOSFETs that are at least partially depleted
during operation and FinFETs including bilayer fins and/or
crystalline oxide.
Inventors: |
Currie; Matthew T. (Brookline,
MA) |
Assignee: |
AmberWave Systems Corporation
(Salem, NH)
|
Family
ID: |
36566573 |
Appl.
No.: |
11/001,166 |
Filed: |
December 1, 2004 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20060113605 A1 |
Jun 1, 2006 |
|
Current U.S.
Class: |
438/164;
257/E21.633; 257/E21.642; 257/E21.703; 257/E27.112; 257/E29.296;
257/E29.298; 438/149; 438/284 |
Current CPC
Class: |
H01L
21/823807 (20130101); H01L 21/823878 (20130101); H01L
21/84 (20130101); H01L 27/1203 (20130101); H01L
29/66795 (20130101); H01L 27/1207 (20130101); H01L
29/78687 (20130101); H01L 29/66772 (20130101); H01L
27/092 (20130101); H01L 29/7849 (20130101); H01L
29/785 (20130101); H01L 29/78681 (20130101) |
Current International
Class: |
H01L
21/00 (20060101); H01L 21/84 (20060101) |
Field of
Search: |
;438/149,152,164,284,412,517,157,283 |
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|
Primary Examiner: Vu; Hung
Attorney, Agent or Firm: Goodwin Procter LLP
Claims
What is claimed is:
1. A method for forming a structure, the method comprising:
providing a substrate having an insulator layer disposed thereon,
and a bilayer disposed in contact with the insulator layer, the
bilayer including a second strained semiconductor layer disposed
over a first strained semiconductor layer; and thereafter forming a
fin field-effect transistor on the substrate by: patterning the
bilayer to define a source region, a drain region, and at least one
fin disposed between the source and the drain regions, forming a
gate dielectric layer at least a portion of which is disposed over
the fin, and forming a gate over the gate dielectric layer portion
disposed over the fin, wherein a strain of the first strained
semiconductor layer is different from a strain of the second
strained semiconductor layer.
2. The method of claim 1, wherein the bilayer comprises at least
one of a group II, a group III, a group IV, a group V, or a group
VI element.
3. The method of claim 1, wherein the first strained semiconductor
layer is tensilely strained.
4. The method of claim 3, wherein the second strained semiconductor
layer is compressively strained.
5. The method of claim 1, wherein patterning the bilayer comprises
removing a portion of the first strained semiconductor layer and a
portion of the second strained semiconductor layer.
6. The method of claim 1, wherein the bilayer comprises a second
semiconductor material disposed over a first semiconductor
material, and the first semiconductor material is different from
the second semiconductor material.
7. The method of claim 1, wherein a thickness of the first strained
semiconductor layer is greater than a thickness of the second
strained semiconductor layer such that the sidewalls of the fin are
primarily defined by the first strained semiconductor layer.
8. The method of claim 1, wherein the second strained semiconductor
layer is tensilely strained.
9. The method of claim 6, wherein the first strained semiconductor
layer is compressively strained.
Description
FIELD OF THE INVENTION
This invention relates generally to semiconductor structures and
particularly to hybrid strained semiconductor-on-insulator
structures.
BACKGROUND
As geometric transistor scaling becomes more difficult and less
effective in providing adequate performance enhancements, there is
an incentive to improve the performance of transistors by enhancing
innate carrier mobility by, e.g., application of strain to the
semiconductor channel material. Although process simplicity is
maintained by the application of one type of strain (or one type of
channel material) for both NMOS and PMOS devices, overall
performance would be greatly improved if it were possible to
enhance the performance of each type of device separately.
Traditionally, this enhancement has been incomplete--one type of
channel material is typically used for both device types, with
selective application of strain to the channel material for each
device.
Devices are advantageously formed on semiconductor-on-insulator
(SOI) substrates. Such substrates offer the benefits of an
insulating substrate, such as reduced parasitic capacitances and
improved isolation.
SUMMARY
The efficacy of the traditional approach for enhancing the
performance of NMOS and PMOS devices may be improved by
additionally customizing the transistor channel materials and their
respective strain levels selectively for NMOS and PMOS devices.
This is particularly true for advanced transistor geometries such
as partially depleted semiconductor-on-insulator (PDSOI) devices,
fully depleted semiconductor-on-insulator (FDSOI) devices, or fin
field-effect transistors (FinFETs).
Although schemes exist to utilize multilayer channel materials
and/or types of strain for transistors (see, e.g., U.S. Ser. Nos.
10/456,926, 10/164,665, 10/177,571, and 10/216,085, and U.S. Pat.
No. 6,730,551, all of which are incorporated herein by reference),
these schemes may not be effective for FDSOI and PDSOI devices when
the total desired channel thickness for each type of device is very
small. Since some such schemes rely on the presence of two channels
in the starting substrate, this bilayer scheme may not allow the
device type that utilizes the top channel for conduction to operate
in fully depleted mode (due to the total thickness of the two
channels being too great for the device to meet the FD maximum
thickness criterion).
In accordance with the invention, NMOS and PMOS devices have
channel layers of different materials and/or types of strain, each
with potentially a very thin thickness. Aspects of the invention
include variations in the starting substrate/channel layer
structure and/or the processes used during device fabrication to
create the final structure. For these solutions, Si--Ge alloys are
used as exemplary materials. In general, it has been demonstrated
that layers of Si--Ge with low Ge contents and/or under tensile
strain are preferred for NMOS devices, and layers with higher Ge
content and/or under compressive strain are preferred for PMOS
devices. Other combinations of materials, including group IV
semiconductors such as alloys of Si, Ge, or SiGe with C; III-V
semiconductors; and II-VI semiconductors may also be suitable.
In an aspect, the invention features a structure including (i) a
semiconductor substrate, a first semiconductor layer including a
first semiconductor material disposed over at least a first portion
of the substrate, and a second semiconductor layer including a
second semiconductor material disposed over at least a second
portion of the substrate; (ii) a first MOSFET disposed on the
substrate and including a first MOSFET channel disposed in a
portion of the first semiconductor layer over a first insulating
material, the first MOSFET channel including the first
semiconductor material; and (iii) a second MOSFET disposed on the
substrate and including a second MOSFET channel disposed in a
portion of the second semiconductor layer over a second insulating
material, the second MOSFET channel including the second
semiconductor material.
The first and second MOSFETs are at least partially depleted during
operation. Moreover, the first MOSFET and/or the second MOSFET may
be fully depleted during operation. Each of the first and second
MOSFETs may be an nMOSFET or a pMOSFET.
The first and/or semiconductor material may include or consist of a
group IV material, a III-V material, and/or a II-VI material.
Specific examples of such materials include silicon, SiGe,
germanium, an array of carbon nanotubes, and mixtures or alloys
thereof; and gallium arsenide, indium arsenide, indium gallium
arsenide, indium phosphide, gallium nitride, indium antimonide,
gallium antimonide, gallium phosphide, and mixtures or alloys
thereof. At least one of the first and second semiconductor
materials may be tensilely strained and/or compressively
strained.
The first semiconductor layer may have a first crystalline
orientation, the second semiconductor layer may have a second
crystalline orientation, and the first crystalline orientation may
be different from the second crystalline orientation. The first
crystalline orientation may be selected from a {100} family of
crystalline planes; the second crystalline orientation may be
selected from a {110} family of crystalline planes.
The first semiconductor layer may have a first crystalline in-plane
rotation, the second semiconductor layer may have a second
crystalline in-plane rotation different from the first crystalline
in-plane rotation.
A crystallographic orientation of the nMOSFET channel may be
parallel to a crystallographic direction selected from any of a
<110> family of crystallographic directions. A
crystallographic orientation of the pMOSFET channel is parallel to
a crystallographic direction selected any of a <100> family
of crystallographic directions.
An insulator layer including the first and second insulating
material may be disposed over the semiconductor substrate, with the
first insulating material being identical or substantially similar
to the second insulating material.
A first insulator layer including the first insulating material may
be disposed over at least the first portion of the substrate, and a
second insulator layer including the second insulator material may
be disposed over at least the second portion of the substrate, such
that the first MOSFET channel is disposed over the first insulator
layer, and the second MOSFET channel is disposed over the second
insulator layer.
The first semiconductor layer may be disposed over a region of the
second semiconductor layer, with the first semiconductor layer
having a first type of strain and a first lattice constant, and the
second semiconductor layer having a second type of strain and the
first lattice constant. Each of the first and second types of
strain may be either of tensile or compressive strain.
The first semiconductor layer may have a first type of strain and a
first lattice constant, and the second semiconductor layer may be
disposed over a region of the first semiconductor layer, the second
semiconductor layer having a second type of strain and the first
lattice constant.
The first MOSFET may include a first gate dielectric layer
(including a first dielectric material) disposed over the first
MOSFET channel and the second MOSFET may include a second gate
dielectric layer (including a second dielectric material) disposed
over the second MOSFET channel. The first and second dielectric
materials may be identical, substantially similar or substantially
different. The first and/or the second dielectric material may
include or consist of at least one of silicon dioxide, silicon
oxynitride, silicon nitride, barium oxide, strontium oxide, calcium
oxide, tantalum oxide, titanium oxide, zirconium oxide, hafnium
oxide, aluminum oxide, lanthanum oxide, yttrium oxide, yttrium
aluminate, lathanum aluminate, lanthanum silicate, yttrium
silicate, hafnium silicate, zirconium silicate, and doped alloys,
undoped alloys, mixtures, and/or multilayers thereof.
The first MOSFET may include a first gate electrode layer
comprising a first conductive material disposed over the first
MOSFET channel, and the second MOSFET may include a second gate
electrode layer comprising a second conductive material disposed
over the second MOSFET channel. The first and second conductive
materials may be identical, substantially similar or substantially
different.
The first and/or second conductive material may include at least
one of doped polycrystalline silicon, doped polycrystalline SiGe,
Al, Ag, Bi, Cd, Fe, Ga, Hf, In, Mn, Nb, Y, Zr, Ni, Pt, Be, Ir, Te,
Re, Rh, W, Mo, Co, Fe, Pd, Au, Ti, Cr, Cu, and doped alloys,
undoped alloys, mixtures, and/or multilayers thereof.
A portion of the first semiconductor layer may be disposed over the
second portion of the substrate, and the second semiconductor layer
may be disposed over the portion of the first semiconductor
layer.
A portion of the second semiconductor layer may be disposed over
the first portion of the substrate and the first semiconductor
layer may be disposed over the portion of the second semiconductor
layer.
The first insulator layer and/or second insulator layer may include
a crystalline oxide layer, which may induces a strain in the first
and second semiconductor layers. The crystalline oxide layer may
include at least one of a multicomponent metal oxide and a
dielectric material having a lattice constant of approximately 5.4
.ANG. and a body-centered cubic structure.
The multicomponent metal oxide may include or consist of one or
more metals selected from the group consisting of Al, Ti, Zr, Hf,
V, Nb, Ta, Cr, Mo, W, and Cu. The multicomponent metal oxide may
include or consist of at least one of barium strontium titanate,
barium strontium zirconate, barium strontium hafnate, lead
titanate, yttrium aluminate, lanthanum aluminate, lead zirconium
titanate, hafnium silicate, zirconium silicate, strontium silicon
oxide, zirconium silicon oxide, hafnium silicon oxide, hafnium
oxide, zirconium oxide, strontium titanate, lanthanum oxide,
yttrium oxide, titanium oxide, barium titanate, lanthanum
aluminate, lanthanum scandium oxide, and/or aluminum oxide. The
dielectric material may include or consist of at least one of
cesium oxide, aluminum nitride, and lanthanum aluminum oxide.
The first insulator layer may include or consist of a first
crystalline oxide that induces a first type of strain in the first
semiconductor layer, and the second insulator layer may include a
second crystalline oxide that induces a second type of strain in
the second semiconductor layer.
The first insulator layer may induce a first strain in the first
semiconductor layer, and the second insulator layer induces a
second strain in the second semiconductor layer.
In another aspect, the invention features a method for forming a
structure, the method including the steps of (i) providing a
semiconductor substrate, (ii) defining a first portion of the
substrate; (iii) defining a second portion of the substrate; (iv)
providing a first insulating material over the first portion of the
substrate; (v) providing a second insulating material over the
second portion of the substrate; (vi) forming a first semiconductor
layer including a first semiconductor material over at least the
first portion of the substrate; (vii) forming a second
semiconductor layer including a second semiconductor material over
at least the second portion of the substrate; (viii) forming a
first MOSFET on the substrate, the first MOSFET including a first
MOSFET channel disposed in a portion of the first semiconductor
layer over the first insulating material, the first MOSFET channel
including the first semiconductor material; and (ix) forming a
second MOSFET on the substrate, the second MOSFET including a
second MOSFET channel disposed in a portion of the second
semiconductor layer over the second insulating material, the second
MOSFET channel including the second semiconductor material. The
first and second MOSFETs are at least partially depleted during
operation.
Defining the first and second portions of the substrate may include
defining a shallow trench isolation region. The first insulating
material may be identical to or substantially the same as the
second insulating material and providing the first and second
insulating materials may include forming an insulator layer over
the substrate.
Forming the first semiconductor layer may include bonding the first
semiconductor layer to the insulator layer. The first semiconductor
layer may be formed over the first and second portions of the
substrate and the second semiconductor layer may be formed over a
second portion of the first semiconductor layer disposed over the
second portion of the substrate. The second portion of the first
semiconductor layer may be thinned prior to forming the second
semiconductor layer. Forming the insulator layer, the first
semiconductor layer, and/or the second semiconductor layer over the
substrate may involve deposition.
Either of the first MOSFET or second MOSFET may be an nMOSFET or a
pMOSFET.
The first semiconductor layer may be formed over the first and
second portions of the substrate. The second semiconductor layer
may be formed over the first semiconductor layer.
A portion of the second semiconductor layer disposed over the first
semiconductor layer over the first portion of the substrate may be
removed.
A regrowth layer may be formed over the first semiconductor layer
disposed over the first portion of the substrate.
Forming the regrowth layer may include providing additional first
semiconductor material and a total thickness of the first
semiconductor layer and the regrowth layer may be approximately the
same as a total thickness of the first semiconductor layer and the
second semiconductor layer in a second portion of the
substrate.
Providing the first and second insulating materials may involve
deposition, and the first insulating material may be different from
the second insulating material. Forming the first and second
semiconductor layers may involve deposition, and the first
semiconductor material may be substantially the same as or
different from the second semiconductor material. At least one of
the first and second insulating materials may include a crystalline
oxide.
The first semiconductor layer may have a thickness selected from a
range of 1-50 nm. The second semiconductor layer may have a
thickness selected from a range of 1-50 nm. For particularly
aggressive FDSOI devices, the first and/or second semiconductor
layer may have a thickness more preferably selected from a range of
1-20 nm, or more preferably 1-10 nm.
In another aspect, the invention features a substrate having an
insulator layer disposed thereon, and a FinFET disposed over the
substrate. The FinFET includes (i) a source region and a drain
region disposed in contact with the insulator layer; (ii) at least
one fin extending between the source and the drain regions and
comprising a bilayer; (iii) a gate disposed above the bilayer,
extending over at least one fin and between the source and the
drain regions; and a gate dielectric layer disposed between the
gate and the fin.
The bilayer may include a second semiconductor material disposed
over a first semiconductor material. The first semiconductor
material and the second semiconductor material may be the same or
different, and each may include or consist of a group IV material,
a III-V material, and/or a II-VI material. The group IV material
may be silicon, SiGe, germanium, an array of carbon nanotubes,
and/or mixtures or alloys thereof. The III-V material may be
gallium arsenide, indium arsenide, indium gallium arsenide, indium
phosphide, gallium nitride, indium antimonide, gallium antimonide,
gallium phosphide, and/or mixtures or alloys thereof.
At least one of the first and second semiconductor materials may be
tensilely strained or compressively strained. The gate dielectric
may be disposed proximate the first semiconductor material and the
second semiconductor material.
In another aspect, the invention comprises a structure including a
substrate having a crystalline oxide layer disposed thereon, and a
FinFET disposed over the substrate. The FinFET includes (i) a
source region and a drain region disposed in contact with the
insulator layer; (ii) at least one fin extending between the source
and the drain regions, the fin comprising a first semiconductor
layer disposed over the crystalline oxide layer; (iii) a gate
disposed above the first semiconductor layer, extending over at
least one fin and between the source and the drain regions; and
(iv) a gate dielectric layer disposed between the gate and the
fin.
In another aspect, the invention comprises a structure including
(i) a substrate; (ii) a first insulator layer disposed over at
least a first portion of the substrate; (iii) a second insulator
layer disposed over at least a second portion of the substrate;
(iv) a first FinFET disposed over the substrate; and (v) a second
FinFET disposed over the substrate. The first FinFET includes (i) a
first source region and a first drain region disposed over and in
contact with the first insulator layer; (ii) a first fin extending
between the first source and the first drain regions, the first fin
including a first semiconductor material disposed on at least one
vertically oriented sidewall of the first fin; (iii) a first gate
disposed above the substrate, extending over the first fin and
between the first source and the first drain regions; and (iv) a
first gate dielectric layer disposed between the first gate and the
first fin. The second FinFET includes (i) a second source region
and a second drain region disposed over and in contact with the
second insulator layer; (ii) a second fin extending between the
second source and the second drain regions, the second fin
including a second semiconductor material disposed on at least one
vertically oriented sidewall of the second fin; (iii) a second gate
disposed above the substrate, extending over the second fin and
between the second source and the second drain regions; and (iv) a
second gate dielectric layer disposed between the second gate and
the second fin. The first semiconductor material has a first
crystalline orientation, the second semiconductor material has a
second crystalline orientation that preferably differs from the
first crystalline orientation.
The first insulator layer may include a first crystalline oxide,
the second insulator layer may include a second crystalline oxide,
the first fin may include the first crystalline oxide, and the
second fin may include the second crystalline oxide.
The first crystalline oxide and the second crystalline oxide may be
substantially different. The first fin and second fin may be
substantially parallel. The first semiconductor material and the
second semiconductor material may be substantially the same. The
first crystalline orientation may be selected from a {100} family
of crystalline planes or a {110} family of crystalline planes. The
first FinFET may include an n-channel device and the second FinFET
comprises a p-channel device.
In another aspect, the invention features a method for forming a
structure, the method including (i) providing a substrate having an
insulator layer disposed thereon, and a bilayer disposed in contact
with the insulator layer, the bilayer including a second
semiconductor layer disposed over a first semiconductor layer; and
(ii) forming a FinFET on the substrate. The FinFET is formed by (i)
patterning the bilayer to define a source region, a drain region,
and at least one fin disposed between the source and the drain
regions, (ii) forming a gate dielectric layer, at least a portion
of the gate dielectric layer being disposed over the fin, and (iii)
forming a gate over the gate dielectric layer portion disposed over
the fin.
The bilayer may include or consist of at least one of a group II, a
group III, a group IV, a group V, or a group VI element. The
bilayer may include a strained semiconductor layer that may be
tensilely strained or compressively strained.
In another aspect, the invention features a structure including (i)
a substrate; (ii) a first FinFET disposed over the substrate; and
(iii) a second FinFET disposed over the substrate. The first FinFET
includes a first semiconductor material having a first crystalline
orientation, the second FinFET includes a second semiconductor
material having a second crystalline orientation, and the first and
second crystalline orientations are different.
In another aspect, the invention features a method for forming a
structure, the method including providing a substrate having an
insulator layer disposed thereon, and a first semiconductor layer
disposed in contact with the insulator layer; and forming a FinFET
on the substrate. The FinFET may be formed by (i) patterning the
first semiconductor layer to define a source region, a drain
region, and at least one fin disposed between the source and the
drain regions, (ii) selectively depositing a second semiconductor
layer over a top surface of at least one fin to form a bilayer,
(iii) forming a gate dielectric layer, at least a portion of the
gate dielectric layer being disposed over the fin, and (iv) forming
a gate over the gate dielectric layer portion disposed over the
fin.
In another aspect, the invention features a method for forming a
structure, the method including providing a substrate having a
crystalline oxide layer disposed thereon, and a first semiconductor
layer disposed in contact with the crystalline oxide layer; and
forming a FinFET on the substrate. The FinFET is formed by (i)
patterning the first semiconductor layer to define a source region,
a drain region, and at least one fin disposed between the source
and the drain regions, (ii) forming a gate dielectric layer, at
least a portion of the gate dielectric layer being disposed over
the fin, and (iii) forming a gate over the gate dielectric layer
portion disposed over the fin.
In another aspect, the invention features a method for forming a
structure, the method including (i) providing a substrate having a
first semiconductor layer disposed thereon; (ii) defining a fin in
the first semiconductor layer, the fin having an aspect ratio; and
(iii) removing top portion of the fin, and thereafter selectively
depositing a second semiconductor layer over the top portion of the
fin so as to preserve the aspect ratio of the fin.
BRIEF DESCRIPTION OF DRAWINGS
FIGS. 1-15 are a series of schematic cross-sectional views of
several alternative semiconductor structure illustrating processes
for fabricating the structures; and
FIGS. 16-33C are schematic cross-sectional and top views of
substrates illustrating a method for fabricating a FinFET.
Like-referenced features represent common features in corresponding
drawings.
DETAILED DESCRIPTION
Referring to FIG. 1, a semiconductor-on-insulator substrate (SOI)
100 includes a semiconductor substrate 110 that itself includes or
consists of a semiconductor material, such as silicon, germanium,
SiGe, silicon carbide, gallium arsenide, indium phosphide, and/or
gallium nitride. An insulator layer 120, e.g., a continuous buried
insulating layer, is disposed over the semiconductor substrate 110.
Insulator layer 120 may include or consist of, for example, silicon
dioxide (SiO.sub.2), silicon nitride (Si.sub.3N.sub.4 or other
compositions), aluminum oxide, magnesium oxide, and/or other
dielectric materials, or may be a multilayer structure including
one or more different materials. The insulator layer 120 may have a
thickness t.sub.0 of, e.g., 50-200 nanometers (nm). For highly
scaled devices, e.g., devices with gate lengths shorter than 100
nm, the insulator layer 120 may be relatively thin, i.e., have a
thickness t.sub.0 of, e.g., 10-50 nm for better control of short
channel effects. In an embodiment, SOI substrate 100 may include a
single insulating substrate (not shown), rather than the
combination of semiconductor substrate 110 and insulator layer 120.
The single insulating substrate may be formed from an insulating
material such as SiO.sub.2, silicon nitride, glass, aluminum oxide,
an organic polymer, plastic, or some combination of materials.
The SOI substrate may be a commercially available substrate that
may be obtained from, e.g., SOITEC Silicon on Insulator
Technologies of Bemin, France.
A first semiconductor layer 130 is disposed over the insulator
layer 120. The first semiconductor layer 130 may include or consist
of a first semiconductor material suitable for use as a channel of
a MOSFET, such as at least one of a group IV material, e.g.,
silicon, SiGe, germanium, or an array of carbon nanotubes; a III-V
material such as gallium arsenide, indium arsenide, indium gallium
arsenide, indium phosphide, gallium nitride, indium antimonide,
gallium antimonide, gallium phosphide; or a II-VI material, and
mixtures or alloys including one or more of the aforementioned
materials. The first semiconductor layer 130 may be strained,
including tensilely or compressively strained, e.g., tensilely or
compressively strained silicon. In some embodiments, the first
semiconductor layer 130 may include approximately 100% Ge, and may
be compressively strained. The first semiconductor layer 130 may
have an initial thickness t.sub.1 of, e.g., 1-50 nm, more
preferably 1-20 nm for fully depleted devices, most preferably 1-10
nm or even 1-5 nm.
The first semiconductor layer 130 may initially be formed on a
handle wafer (not shown) and then bonded to the insulator layer
120. For example, a handle wafer may include a Si.sub.1-xGe.sub.x
layer with x>0. A silicon layer formed over this
Si.sub.1-xGe.sub.x layer will be tensilely strained, and remains
tensilely strained after being bonded to the insulator layer 120 to
form the first semiconductor layer 130. A germanium layer formed
over this Si.sub.1-xGe.sub.x layer will be compressively strained,
and remains compressively strained after being bonded to the
insulator layer 120 to form the first semiconductor layer 130.
Alternatively, strain in the first semiconductor layer may arise
from mechanical deformation of the handle wafer or from thermal
mismatch with the handle wafer. For example, the handle wafer may
be mechanically biaxially or uniaxially strained by bending or
heated to elevated temperature prior to bonding to insulator layer
120. After the handle wafer is removed, first semiconductor layer
130 will remain strained. See, for example, U.S. Ser. No.
10/456,103, filed Jun. 6, 2003, incorporated herein in its
entirety.
A conductive layer (not pictured) may be disposed beneath insulator
layer 120. This conductive layer may be used in subsequently formed
devices as a ground plane or as a second gate in, e.g., in a planar
double-gate transistor. This conductive layer may include the same
material as may be used for a gate electrode, e.g., doped
polycrystalline silicon, doped polycrystalline SiGe, Al, Ag, Bi,
Cd, Fe, Ga, Hf, In, Mn, Nb, Y, Zr, Ni, Pt, Be, Ir, Te, Re, Rh, W,
Mo, Co, Fe, Pd, Au, Ti, Cr, Cu, and doped or undoped alloys, or
mixtures or multilayers thereof.
During device fabrication, a first portion 140 of the substrate 100
and a second portion 150 of the substrate 100 may be defined as
shown in FIG. 2. The first and second substrate portions 140, 150
may be defined by, e.g., the formation of a shallow trench
isolation (STI) region 160. The STI region 160 may be formed by
methods known in the art, e.g., as described in co-pending U.S.
Ser. No. 10/794,010, publication No. 2004/0173812 A1, incorporated
by reference herein in its entirety.
A first MOSFET may be fabricated on the first portion 140 of the
substrate 100 and a second MOSFET may be fabricated on the second
portion 150 of the substrate 100 as follows. After STI 160 has been
defined, a first portion 170 of the first semiconductor layer 130
disposed over the first portion of the substrate 100 may covered by
a mask 180. The mask 180 may be formed from a masking material
selected to be stable during the formation of a second layer
comprising a second material over the second portion of the
substrate. Moreover, the masking material is selected such that it
may be selectively removed with respect to the second semiconductor
layer, as described below. The masking material may include or
consist of a dielectric material, such as silicon dioxide, silicon
oxynitride, or silicon nitride.
In an embodiment, the first MOSFET is an nMOSFET and the second
MOSFET may be a pMOSFET. In another embodiment, the first MOSFET is
a pMOSFET and the second MOSFET may be an nMOSFET. In yet another
embodiment, both the first and second MOSFETs are both nMOSFETs or
pMOSFETs.
Mask 180 may be defined after the completion of STI formation.
Alternatively, mask 180 may include masking material used to
protect those regions of the first semiconductor layer that are not
removed during STI formation; after STI formation, the masking
material may be selectively removed from the second portion of the
substrate where the pMOSFET will be formed, thereby exposing the
portion of the first semiconductor layer disposed over the second
portion of the substrate. Masking material used during STI
formation may be, for example, a silicon nitride
chemical-mechanical polishing (CMP) stop layer 182 disposed over a
pad oxide layer 184.
Referring to FIG. 2 as well as to FIG. 3, a second semiconductor
layer 200 may be formed over an exposed surface of a second portion
210 of the first semiconductor layer 130 that is disposed over the
second portion 150 of the substrate 100. The second semiconductor
layer 200 may include or consist of a material suitable for use as
a channel of a MOSFET, e.g., a group IV material such as silicon,
SiGe, germanium, or an array of carbon nanotubes; a III-V material
such as gallium arsenide, indium arsenide, indium gallium arsenide,
indium phosphide, gallium nitride, indium antimonide, gallium
antimonide, or gallium phosphide; and a II-VI material, or mixtures
or alloys including one or more of the aforementioned materials.
The second semiconductor layer 200 may be strained, including
tensilely or compressively strained, e.g., tensilely or
compressively strained silicon.
The second semiconductor layer 200 may be formed by a deposition
process, such as chemical-vapor deposition (CVD) or atomic layer
deposition (ALD). CVD includes the introduction of multiple
reagents into a reactor simultaneously. ALD includes the sequential
introduction of multiple reagents into a reactor, including, but
not limited to, atomic layer epitaxy, digital chemical vapor
deposition, pulsed chemical vapor deposition, and other like
methods.
A thickness t.sub.2 of the second semiconductor layer 200 may be
selected to be thick enough to enable carrier conduction, e.g., in
the channel of a subsequently formed transistor, while preferably
thin enough to support fully depleted device operation. The second
semiconductor layer 200 may have a thickness t.sub.2 of, e.g., 1-50
nm, more preferably 1-20 nm, most preferably 1-10 nm or even 1-5 nm
for fully depleted devices.
In some embodiments, a total thickness t.sub.3 of the first
semiconductor layer 130 initial thickness t.sub.1 and the second
semiconductor layer thickness t.sub.2 may be too great to allow
fully depleted operation of devices formed on the second
semiconductor layer 200. It may be advantageous, therefore, to
reduce the initial thickness t.sub.1 of at least that portion of
the first semiconductor layer 130 disposed over the second portion
150 of the substrate, prior to the formation of the second
semiconductor layer 200. The initial thickness t.sub.1 of the first
semiconductor layer 130 may be selectively reduced over the second
portion 150 of the substrate by etching, e.g., by reactive ion
etching (RIE) or by an in-situ etch prior to deposition in the
deposition tool. For example, in an embodiment in which the first
semiconductor layer 130 is silicon, the exposed portion 210 of the
first semiconductor layer may be thinned in a chlorine-containing
ambient including, e.g., hydrogen chloride or chlorine, to a
reduced thickness t.sub.4 of, e.g., 1-10 nm. The reduced thickness
t.sub.4 is thin enough such that the total thickness t.sub.3 of
reduced thickness t.sub.4 and second semiconductor layer thickness
t.sub.2 will enable fully depleted device operation. The reduced
thickness t.sub.4 may be thick enough such that the remaining
portion of the first semiconductor layer 130 does not agglomerate
during subsequent thermal processing prior to and including the
deposition of second semiconductor layer 200. Subsequently, the
second semiconductor, layer 200 may be deposited over the exposed
portion 210 of the first semiconductor layer 130, such that the
total thickness t.sub.3 of the second semiconductor layer 200 and
the thinned first semiconductor layer 130 is less than 50 nm, i.e.,
a sufficiently small thickness for the formation of fully depleted
devices. In some embodiments, the total thickness t.sub.3 of the
reduced first semiconductor layer 130 thickness t.sub.4 and the
second semiconductor layer 200 thickness t.sub.2 may be
approximately the same as t.sub.1, i.e., the initial thickness of
the first semiconductor layer 130.
In practice, the total thickness t.sub.3 of the first semiconductor
layer 130 and the second semiconductor layer 200 is preferably 0.25
to 0.7 times a gate length of a transistor to be formed over the
first and second semiconductor layers 130, 200. If the portion of
the first semiconductor layer 130 disposed over the second portion
of the substrate is completely removed, the thickness t.sub.2 of
the second semiconductor layer 200 may be 0.25 to 0.7 times a gate
length of a transistor formed solely over the second semiconductor
layer 200. Similarly, in a transistor formed solely over the first
semiconductor layer 130, the initial thickness t.sub.1 of the first
semiconductor layer preferably may be 0.25 to 0.7 times a gate
length of that transistor. Selection of a semiconductor layer
thickness in the range of 0.25 to 0.7 times a transistor gate
length may be preferable for improved operation of the transistor,
as this relationship between the semiconductor layer thickness and
the transistor gate length may enable fully depleted behavior in
the transistor.
In an embodiment, the first semiconductor layer 130 includes
relaxed silicon. A compressively strained second semiconductor
layer 200 may be formed by thinning the first semiconductor layer
130 and depositing the second semiconductor layer 200 such that the
second semiconductor layer 200 includes compressively strained
SiGe.
In another embodiment, the first semiconductor layer 130 includes
strained silicon. To obtain a compressively strained second
semiconductor layer 200, the first semiconductor layer 130 may be
thinned and the second semiconductor layer 200 deposited, the
second semiconductor layer 200 including compressively strained
Si.sub.1-xGe.sub.x material having a Ge content x greater than that
of the Si.sub.1-xGe.sub.x layer of a handle wafer over which the
first semiconductor layer 130 has been formed.
More generally, a compressively strained second semiconductor layer
200 may be formed from a material having a relaxed lattice constant
greater than the strained lattice constant to which the first
semiconductor layer 130 is strained, i.e., the second semiconductor
material may have a relaxed lattice constant greater than that of
the handle wafer on which the first semiconductor layer has been
formed (and which induced strain in the first semiconductor layer).
The deposition of such a second semiconductor material on the
tensilely strained first semiconductor layer will cause the second
semiconductor layer 200 to be compressively strained to the same
lattice constant to which the first semiconductor layer 130 is
strained.
As an example, consider the lattice constants of three different
materials. The lattice constant of relaxed silicon is less than the
lattice constant of relaxed Si.sub.0.5Ge.sub.0.5, which is less
than the lattice constant of relaxed germanium. Strained silicon
grown on relaxed Si.sub.0.5Ge.sub.0.5 will have a lattice constant
of the relaxed Si.sub.0.5Ge.sub.0.5 and will be tensilely strained.
Strained germanium grown on relaxed Si.sub.0.5Ge.sub.0.5 will also
have the lattice constant of the relaxed Si.sub.0.5Ge.sub.0.5, but
will be compressively strained. Therefore, if the first
semiconductor layer is substantially 100% strained silicon with the
lattice constant of relaxed Si.sub.0.5Ge.sub.0.5, the second
semiconductor layer, in order to be compressively strained, has a
lattice constant greater than that of relaxed
Si.sub.0.5Ge.sub.0.5.
In an alternative embodiment, the first semiconductor layer 130 may
be a compressively strained germanium layer suitable for the
formation of a pMOSFET. Then, the first semiconductor layer 130 may
be thinned over a first portion 140 of the substrate 100 over which
an nMOSFET will be formed. The second semiconductor material may
include a material suitable for use as the channel of an nMOSFET,
such as a group IV material, e.g., silicon, SiGe, germanium, or an
array of carbon nanotubes; a III-V material such as gallium
arsenide, indium arsenide, indium gallium arsenide, indium
phosphide, gallium nitride, indium antimonide, gallium antimonide,
or gallium phosphide; or a II-VI material, and mixtures or alloys
including one or more of the aforementioned materials.
Referring to FIG. 4, a gate dielectric layer 220 may be formed over
the first and second semiconductor layers 130, 200 by, e.g.,
deposition or growth. The gate dielectric layer 220 may be formed
by one or more of a number of techniques, such as molecular beam
epitaxy (MBE), oxidation, nitridation, CVD, ALD, or a combination
of these or other methods. The gate dielectric layer may include or
consist of a dielectric material such as, for example, silicon
dioxide (SiO.sub.2), silicon oxynitride (SiO.sub.xN.sub.y), silicon
nitride (Si.sub.3N.sub.4 or other compositions), barium oxide
(BaO), strontium oxide (SrO), calcium oxide (CaO), tantalum oxide
(Ta.sub.2O.sub.5), titanium oxide (TiO.sub.2), zirconium oxide
(ZrO.sub.2), hafnium oxide (HfO.sub.2), aluminum oxide
(Al.sub.2O.sub.3), lanthanum oxide (La.sub.2O.sub.3), yttrium oxide
(Y.sub.2O.sub.3), yttrium aluminate, lathanum aluminate, lanthanum
silicate, yttrium silicate, hafnium silicate, zirconium silicate,
and doped or undoped alloys, mixtures or multilayers, thereof.
Alternatively, the first and second gate dielectric layers may
include first and second dielectric materials that are identical or
substantially similar. Essentially, a single gate dielectric layer
220 may be utilized over the first and second substrate regions
140, 150.
A gate electrode layer 230 may be formed over the gate dielectric
layer by, e.g., CVD or ALD, and subsequently patterned to define a
gate. The gate electrode layer 230 may include or consist of a
suitably conductive material such as, for example, doped
polysilicon, doped polycrystalline SiGe, Al, Ag, Bi, Cd, Fe, Ga,
Hf, In, Mn, Nb, Y, Zr, Ni, Pt, Be, Ir, Te, Re, Rh, W, Mo, Co, Fe,
Pd, Au, Ti, Cr, Cu, and doped or undoped alloys, mixtures or
multilayers thereof. Gate electrode layers of different materials
and/or compositions may be utilized over the first and second
substrate regions 140, 150, i.e., a first electrode layer including
a first conductive material may be formed over the first substrate
region 140, a second electrode layer including a second conductive
material may be formed over the second substrate region 150, and
the first and second conductive materials may be substantially
different.
Alternatively, the first gate electrode layer may include a first
conductive material, the second gate dielectric layer may include a
second conductive material, and the first and second conductive
materials may be identical or substantially similar. Essentially, a
single gate electrode layer 230 may be utilized over both first and
second substrate regions.
The use of two different channel materials may facilitate the use
of a single midgap metal gate for nMOSFET and pMOSFET devices,
e.g., a gate electrode having a workfunction between that of
n.sup.+ polysilicon (approximately 4.2 eV) and p.sup.+ polysilicon
(approximately 5.2 eV) and preferably approximately 4.4-4.9 eV,
such as titanium nitride (TiN), tantalum nitride (TaN), tantalum
silicon nitride (TaSiN), tungsten (W), molybdenum (Mo), titanium
(Ti), tantalum (Ta), polycrystalline carbon (C), or silicides of
nickel or other metals (e.g., NiSi), thus making device fabrication
less complex in comparison to the use of two different gate
electrodes.
Portions of the gate electrode layer 230 and gate dielectric layer
220 may be removed as shown in FIGS. 4 and 5 by, e.g., etching to
define first and second gate electrodes 240a, 240b disposed over
the first and second semiconductor layers 130, 200, respectively.
Sidewall spacers 250 may be defined proximate the gate electrodes
240a, 240b. Source and drain regions 260 may be formed for nMOSFET
and pMOSFET devices by, e.g., implanting or diffusing appropriate
dopants proximate the gate electrodes 240a, 240b, as is known in
the art.
The source and drain regions 260 may also include a semiconductor
material (which may be different from the semiconductor material
disposed in nMOSFET channel 270a and pMOSFET channel 270b) defined
in the first and second semiconductor layers 130, 200,
respectively, beneath the gate electrodes 240a, 240b. Such a
source/drain material may be formed by deposition (e.g., CVD or
ALD), which may be preceded by an etch that removes at least some
of the preexisting material present in the source and drain regions
260. The source/drain material may induce strain in the device
channels 270a, 270b because of a difference in lattice constant
and/or coefficient of thermal expansion between the source/drain
material and the channel materials disposed beneath the gate
electrodes 240a, 240b. The source/drain material may also serve to
decrease series or contact resistance in the nMOSFET and pMOSFET
devices. The source/drain material may also enable silicidation of
the source and drain regions, as the initial thickness t.sub.1 of
first semiconductor layer 130 and/or thickness t.sub.3 (the reduced
thickness t.sub.4 of the first semiconductor layer 130 in addition
to thickness t.sub.2 of second semiconductor layer 200) present in
the source and drain regions may not be sufficient to enable
formation of a low-resistivity silicide.
A self-aligned silicide (salicide) may be formed in source and
drain regions 260 and optionally on top of first and second gate
electrodes 240a, 240b as follows. A conductive layer is formed over
the substrate 100. For example, a metal such as titanium, platinum,
zirconium, cobalt, nickel, or alloys, mixtures, or multilayers
thereof is deposited by, e.g., CVD or sputtering, with the
conductive layer having a thickness of, e.g., 50-200 .ANG.. In some
embodiments, additional semiconductor material is formed over
source and drains 260 regions, and optionally over the gate
electrodes 240a, 240b, prior to the formation of the conductive
layer, to define raised source and drain regions. An anneal is
performed to react the conductive layer with source and drain
regions 260 and optionally with tops of first and second gate
electrodes 240a, 240b. Anneal parameters may be, for example,
400-800.degree. C. for 1-120 seconds. Unreacted portions of the
conductive layer disposed directly over insulator material, such as
sidewall spacers 250, are removed by a chemical strip. A suitable
chemical strip is a solution including
H.sub.2SO.sub.4:H.sub.2O.sub.2 at a ratio of 3:1. A second anneal
may be performed to further lower resistivity of the salicide. The
second anneal parameters may be, for example, 600-900.degree. C.
for 1-120 seconds.
An nMOSFET 280 includes channel 270a disposed in a portion of the
first semiconductor layer 130 over a first insulating material
120a, such that the nMOSFET channel 270a includes the first
semiconductor material. A pMOSFET 290 includes channel 270b
disposed in a portion of the second semiconductor layer 200 over a
second insulating material 120b, such that the pMOSFET channel 270b
includes the second semiconductor material. In some embodiments,
insulator layer 120 including the first and second insulating
materials is disposed across the semiconductor substrate 110, and
the first and second insulating materials may be substantially
similar. In other embodiments, a first insulator layer including
the first insulating material is disposed over a first portion of
the semiconductor substrate 110, a second insulator layer including
the second insulating material is disposed over a second portion of
the semiconductor substrate 110, and the first and second
insulating materials are substantially different. This embodiment
is described in greater detail with respect to FIGS. 12-15.
The channels of the nMOSFET and pMOSFET may include one or more
channel dopants, e.g., boron, arsenic, antimony, phosphorous, or
indium. Such channel dopants may be of a type opposite the dopant
type present in the source and drain regions of a particular
device, and may influence control of the device threshold voltage.
For example, the nMOSFET may include arsenic doping in the source
and drain regions and boron in the channel region. These dopants
may be present at a fairly low level, e.g., at a level less than
10.sup.16-10.sup.17 cm.sup.-3. In an embodiment, these channel
dopants may be present at a level less than 10.sup.15 cm.sup.3.
In some embodiments, the nMOSFET and the pMOSFET devices 280, 290
have different gate dielectric compositions and/or different gate
electrode compositions. This may be achieved by the utilization of
gate dielectric layers of different materials and/or compositions
over the first and second substrate regions 140, 150, i.e., a first
dielectric layer 220a including a first dielectric material may be
disposed over the first substrate region 140, a second dielectric
layer 220b including a second dielectric material may be disposed
over the second substrate region 150, and the first and second
dielectric materials may be substantially different. Two different
types of dielectric layers 220a, 220b and gate electrode layers may
be formed by the use of a "dummy gate" process. Here, the nMOSFET
and pMOSFET devices 280, 290 are formed as described above, with
the use of single gate dielectric layer 220. After the formation of
sidewall spacers 250, a thick oxide layer may be formed over the
nMOSFET and pMOSFET devices 280, 290 and planarized by, e.g., CMP,
to expose top portions of the first and second gate electrodes
240a, 240b. A mask may be formed over, e.g., the nMOSFET device 280
to protect the gate electrode 240a. The gate electrode 240b and the
underlying gate dielectric material of the pMOSFET 290 may be
removed by, e.g., a wet etch. A second gate dielectric layer 220b
including a second dielectric material and a second gate electrode
240b may be defined for the pMOSFET 290. The second dielectric
material may be substantially different from the first dielectric
material. The second gate electrode material may be substantially
different from the first gate electrode material. A second
planarization step may be performed to remove residual materials
such as masks, as well as portions of the second gate dielectric
material and second gate electrode material that are not needed for
the second gate dielectric layer and the second gate electrode.
Finally, the thick oxide is removed by, e.g., a wet etch.
As used herein, a "fully depleted" SOI device is fully depleted
between the channel and the underlying insulating layer when the
gate voltage is equal to the device's threshold voltage. The region
between the channel and the underlying insulating layer is
considered to be fully depleted if it is substantially free of
charge carriers. The nMOSFET may be fully depleted during
operation, and the pMOSFET may be fully depleted during
operation.
During device operation, both the pMOSFET and the pMOSFET may have
similar body thicknesses, i.e., similar total thicknesses of layers
disposed beneath the gate, and both device types may have carrier
depletion regions beneath the respective channels that extend to
the underlying insulator layer.
Partially depleted on-insulator devices may be formed over
particular regions of the substrate by modification of the above
technique. More specifically, the body thickness, i.e., the
thickness of the first or second semiconductor layers, may be
sufficiently increased for certain devices to allow partially
depleted operation. As used herein, a "partially depleted" SOI
device is not fully depleted between the channel and the underlying
insulating layer when the gate voltage is equal to the device's
threshold voltage. When a region is not fully depleted, it is not
substantially free of charge carriers.
An alternative starting material may be used to form the
semiconductor structure having a first material disposed over a
first portion of the substrate and a second material disposed over
a second portion of the substrate, as illustrated in FIG. 3. A
bilayer semiconductor layer structure 300 may be formed over the
insulator layer 120 disposed on substrate 110 as shown in FIG. 6.
This bilayer 300 includes first semiconductor layer 130 and second
semiconductor layer 200 disposed over the first semiconductor layer
130. The second semiconductor layer 200 may include any material
particularly suitable for either nMOSFET or pMOSFET operation. For
example, for use in a pMOSFET device, the bilayer structure may be
thin, having a total thickness t.sub.5 of, e.g., 1-50 nm. The
second semiconductor layer 200 may be any material suitable for use
as a channel of a pMOSFET and may include, e.g., unstrained or
strained germanium. The second semiconductor layer 200 may
constitute the majority of the thickness of the bilayer 300. The
underlying first semiconductor layer 130 may include any material
suitable for use as a channel of an nMOSFET, e.g., unstrained or
strained silicon.
The presence of a thin first semiconductor layer under the second
semiconductor layer in a pMOSFET will not disturb operation of the
pMOSFET, and may serve as a template for epitaxial formation of a
thicker regrowth layer including the first semiconductor material,
as described below, for use as a channel of an nMOSFET.
First and second portions 140, 150 of the substrate 100 may be
defined as shown in FIG. 7 by, e.g., the formation of STI region
160, as described above with reference to FIG. 2.
After STI 160 has been defined, a first portion 310 of the second
semiconductor layer 200 may be exposed while a second portion 320
of the second semiconductor layer 200 (disposed over the second
portion 150 of the substrate 100) is covered by mask 180. The mask
180 may be formed from a masking material selected such that the
material is stable during the removal of the first portion 310 of
the second semiconductor layer 200 and the formation of a regrowth
layer comprising a first material over the first portion 140 of the
substrate 100. Moreover, the masking material is desirably selected
such that it may be selectively removed with respect to the first
semiconductor layer 130, as described below. The masking material
may include a dielectric material, such as silicon dioxide, silicon
oxynitride, or silicon nitride.
Mask 180 may be defined after the completion of STI 160 formation.
Alternatively, mask 180 may include masking material used to
protect those regions of the bilayer that are not removed during
STI formation; after STI formation, the masking material may be
selectively removed from the first portion of the substrate where
the nMOSFET will be formed, thereby exposing the portion of the
second semiconductor layer disposed over the first portion of the
substrate. Masking material used during STI formation may be, for
example, a silicon nitride CMP stop layer 182 disposed over pad
oxide layer 184.
The exposed portion 310 of the second semiconductor layer 200
disposed over the first portion 140 of the substrate 100 may be
removed as shown in FIGS. 7 and 8 by, e.g., RIE or by an in-situ
etch prior to regrowth in the deposition tool. After the removal of
the exposed second semiconductor layer portion 310, a first portion
of the first layer 130 will be exposed. A regrowth layer 330
including the first material may be formed over the exposed first
portion of the first layer 130, thus providing a single layer
including the first material for subsequent use as a channel for an
nMOSFET device.
After the regrowth of the first semiconductor layer, one obtains
the same structure illustrated in FIG. 3. An nMOSFET may be formed
over the first portion 140 of the substrate 100 and a pMOSFET may
be formed over the second portion 150 of the substrate 100, as
described above with reference to FIGS. 4 and 5.
In both embodiments, a final structure may include nMOSFET and
pMOSFET devices, with each type of device having a channel of
approximately the same thickness, one type having a single-layer
channel and the other having a bilayer channel.
In an embodiment, regrowth layer 330 includes a semiconductor
material included in first and second semiconductor layers 130,
200. In this case, deposition of regrowth layer 330 may result in
first and second portions 140, 150 of substrate 100 both including
bilayers being different combinations of materials. Referring to
FIGS. 7 and 8B, in another embodiment, exposed second semiconductor
layer portion 310 is not completely removed prior to deposition of
regrowth layer 330. In this case, deposition of regrowth layer 330
in first portion 140 of substrate 100 may result in a trilayer
structure 335 including a portion of first semiconductor layer 130,
a portion of second semiconductor layer 200, and regrowth layer
330. A total thickness of this trilayer structure may be
approximately the same as the thickness of the bilayer structure
present in second portion 150 of substrate 100.
In an embodiment, after bilayer 300 or trilayer structure 335 is
formed over first or second portions 140, 150 of substrate 100, an
anneal is performed to interdiffuse the bilayer or trilayer
structure. The anneal step may be performed at a range of suitable
temperatures and times, e.g., 800-1100.degree. C. for 1 second-1
hour. The anneal step causes the layers in the bilayer or trilayer
to interdiffuse to form a single layer having a composition at
least slightly different from the composition of the layers of the
bilayer or trilayer structure. This may be desirable in order to
eliminate sharp offsets in the band structure of the bilayer or
trilayer structure that may affect device performance.
In an alternative embodiment, a crystalline epitaxial oxide layer
disposed over semiconductor substrate selectively induces strain in
first and second semiconductor layers disposed over the crystalline
oxide layer.
Referring to FIG. 9, a crystalline epitaxial oxide layer 400 is
disposed over semiconductor substrate 110. The crystalline oxide
layer 400 includes a material having a lattice parameter selected
to exert an appropriate strain, both in terms of level and
direction, on first and second semiconductor layers 130, 200.
For example, crystalline oxide layer 400 may include or consist of
a material such as strontium titanate (SrTiO.sub.3 or other
compositions) or lanthanum aluminum oxide (LaAlO.sub.3 or other
compositions), having a lattice parameter approximately equal to
that of Si.sub.0.5Ge.sub.0.5 that exerts a tensile strain on Si and
a compressive strain on Ge. Therefore, a first semiconductor layer
including or consisting of Si disposed on crystalline oxide layer
400 may be tensilely strained, and a second semiconductor layer
including or consisting of Ge disposed on crystalline oxide layer
400 may be compressively strained.
The crystalline oxide layer 400 may include or consist of a
multicomponent metal oxide such as a pervoskite-type oxide having
the formula ABO.sub.3 with B including at least one acid oxide
containing a metal such as Al, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, or
Cu, and A including at least one additional cation having a
positive formal charge of from about 1 to about 3. Examples of such
cations include cesium, strontium, barium, rubidium, yttrium,
scandium, and lanthanum. Thus, examples of appropriate
multicomponent metal oxides include but are not limited to: barium
strontium titanate, barium strontium zirconate, barium strontium
hafnate, lead titanate, yttrium aluminate, lanthanum aluminate,
lead zirconium titanate, hafnium silicate, zirconium silicate, and
rare earth-doped silicates.
Other examples of materials suitable for use as crystalline oxide
layer 400 include metal silicate materials, such as strontium
silicon oxide (SrSiO.sub.4), zirconium silicon oxide (ZrSiO.sub.4),
and hafnium silicon oxide (HfSiO.sub.4), hafnium oxide (HfO.sub.2),
zirconium oxide (ZrO.sub.2), strontium titanate (SrTiO.sub.3),
lanthanum oxide (La.sub.2O.sub.3), yttrium oxide (Y.sub.2O.sub.3),
titanium oxide (TiO.sub.2), barium titanate (BaTiO.sub.3),
lanthanum aluminate (LaAlO.sub.3), lanthanum scandium oxide
(LaScO.sub.3) and aluminum oxide (Al.sub.2O.sub.3).
Other options crystalline oxide layer 400 include any of several
dielectric materials having lattice constants and structures
similar to that of silicon. For example, cesium oxide (CeO.sub.2),
aluminum nitride (AlN) and lanthanum aluminum oxide (LaAlO.sub.3)
all have suitable lattice constants and crystalline structures.
Crystalline oxide layer 400 may be chosen to be deliberately
lattice-mismatched to semiconductor substrate 110 to provide a
lattice constant different therefrom for subsequent layer
deposition. The crystalline oxide layer 400 material may be
selected such that a major crystallographic plane of crystalline
oxide layer 400 parallel to a surface of semiconductor substrate
110 differs from the crystallographic plane of that surface. This
configuration may facilitate the desired lattice match or mismatch
between crystalline oxide layer 400 and semiconductor substrate
110. For example, semiconductor substrate 110 may include silicon,
SiGe, or germanium with a {100} surface, and crystalline oxide
layer 400 may include an aforementioned material with a {200},
{110}, or {111} (i.e., not {100}) crystallographic plane parallel
to the surface of semiconductor substrate 110. Such a combination
may provide an effective in-plane lattice constant of crystalline
oxide layer 400 suitable for lattice match or mismatch with
semiconductor substrate 110 or subsequently deposited layers. This
combination may also facilitate the formation of subsequently
deposited layers, e.g., first and second semiconductor layers 130,
200, having a crystallographic orientation (i.e., surface
crystallographic plane) different from a surface crystallographic
orientation of semiconductor substrate 110 and/or having an
in-plane rotation of the surface crystallographic plane different
from that of semiconductor substrate 110. Such changes in
crystalline orientation or rotation may result in enhanced carrier
mobilities in devices subsequently fabricated on first and second
semiconductor layers 130, 200.
Crystalline oxide layer 400 may have a single composition
throughout its thickness. Alternatively, the composition of
crystalline oxide layer 400 may vary throughout its thickness. For
example, with a ternary oxide such as lanthanum aluminum oxide, the
lanthanum content may gradually increase, thus increasing the
lattice constant of the layer. Such grading of composition in
crystalline oxide layer 400 may help prevent formation of defects
due to lattice mismatch between crystalline oxide layer 400 and
semiconductor substrate 110. Alternatively, crystalline oxide layer
400 may include multiple crystalline oxide layers, each having a
different composition.
Crystalline oxide layer 400 may be formed by deposition, e.g., by
CVD or ALD. The crystalline oxide layer 400 has a typical thickness
t.sub.6 of about 10-500 nm. Preferably, crystalline oxide 400 is
thick enough to support subsequent deposition of first and second
semiconductor layers 130, 200. Since many crystalline oxides have
dielectric constants higher than that of silicon dioxide, a thick
crystalline oxide 400 may be desirable to decrease capacitance.
In another embodiment, a thin amorphous layer (not shown) is formed
on a top or a bottom surface of crystalline oxide 400 to prevent
defect formation at an interface between crystalline oxide layer
400 and semiconductor substrate 110 or between the crystalline
oxide layer and either of the first and second semiconductor layers
130, 200. The thin amorphous layer may include an oxide, e.g.,
SiO.sub.2, SiGeO.sub.2, and/or GeO.sub.2. The thin amorphous layer
may be formed by a thermal treatment after the formation of
crystalline oxide 400, optionally in an ambient including
oxygen.
Referring to FIG. 10, in another embodiment, a capping layer 410
may be formed on a top surface of crystalline oxide 400. The
capping layer 410 may protect the crystalline oxide layer 400 from
various wet chemical treatments of the substrate prior to the
formation of first and second semiconductor layers. If capping
layer 410 includes a material that can be removed selectively with
respect to the underlying crystalline oxide 400, e.g., silicon
nitride or a semiconductor, crystalline oxide 400 can be protected
during subsequent masking and mask removal steps, e.g., those steps
described below.
Referring to FIGS. 11-13, after the formation of crystalline oxide
400 and, optionally, capping layer 410, STI regions 160 may be
defined with use of mask 180 as described above with reference to
FIG. 2. Similarly, first semiconductor layer 130 may be defined
over a region of the crystalline oxide layer disposed over the
first portion 140 of the substrate and second semiconductor layer
200 may be defined over a region of crystalline oxide layer
disposed over the second portion 150 of the substrate. First and
second semiconductor layers 130, 200 may be chosen to be
deliberately lattice-mismatched to the crystalline oxide layer 400
such that the semiconductor layers are under tensile or compressive
strain. This strain may be biaxial in nature. In an embodiment, the
lattice structure of crystalline oxide layer 400 is such that at
least one of first and second semiconductor layers 130, 200 is
primarily uniaxially strained in-plane.
Preferably, if optional capping layer 410 is initially disposed
over the top surface of the crystalline oxide layer 400, then layer
410 is removed during the formation of first and second
semiconductor layers 130, 200, e.g., by a wet or dry etch prior to
deposition.
Subsequently, an nMOSFET may be formed, including a channel
disposed in a portion of the first semiconductor layer 130 and a
pMOSFET may be formed, including a channel disposed in a portion of
the second semiconductor layer 200.
The embodiments described above include a continuous insulator
layer disposed across a wafer. In some embodiments, the insulator
layer may be discontinuous. Moreover, the insulator layer may
include a first insulator layer including a first insulating
material disposed over at least a first portion of the substrate
and a second insulator layer including a second insulating material
disposed over at least a second portion of the substrate. For
example, different crystalline oxides may be formed selectively in
NMOS and PMOS regions. An insulator layer such as SiO.sub.2 may
only be present below one or more device channel regions.
Referring to FIG. 14A, semiconductor substrate 110 may be used as a
starting material. STI regions 160 extending into semiconductor
substrate 110 may be defined as described above with reference to
FIG. 2. After STI 160 is defined, the first portion 140 of the
substrate 110 is exposed and the second portion 150 of the
substrate is covered by mask 180, e.g., a mask defined after the
completion of STI formation. Alternatively, mask 180 may include
masking material used to protect those regions of the substrate 110
that are not exposed during STI formation; after STI formation, the
masking material may be selectively removed from the first portion
140 of the substrate 110 where the nMOSFET will be formed, thereby
exposing the first portion 140 of the substrate. Masking material
used during STI formation may be, for example, silicon nitride CMP
stop layer 182 disposed over pad oxide layer 184.
Referring to FIG. 14B, a first insulator layer 500 including a
crystalline oxide material (and also referred to herein as "a first
crystalline oxide layer") may be defined over the first portion 140
of substrate 110. The first insulator layer 500 may be formed by
deposition, e.g., by CVD or ALD, or a similar technique. Typically,
the crystalline oxide layer is not formed selectively; rather, it
is formed over the first portion 140 of the substrate 110 as well
as over the mask 180 disposed over the second portion 150 of the
substrate 110. Therefore, it may be preferable to mask off a region
of the first insulator layer 500 disposed over the first portion
140 of the substrate 110, prior to removal of the remainder of the
crystalline oxide material and mask disposed over the second
portion 150 of the substrate 110. Alternatively, the first
insulator layer 500 and a capping nitride layer (not shown) may be
deposited over substantially the entire substrate 110. Then a CMP
step may be performed to remove the portions of the capping nitride
layer and first insulator layer 500 disposed over the second
portion 150 of the substrate 110, stopping at a surface of the mask
180 disposed over the second region 150. Any residual nitride from
the capping layer remaining over both the first and second portions
140, 150 of the substrate 110 may then be removed. This process
also results in the formation of first insulator layer 500 over the
first portion 140 of the substrate 110.
Referring to FIGS. 14B and 14C, after the formation of the first
insulator layer 500, the mask 180 is removed, thereby exposing a
top surface of the second portion 150 of the substrate. The mask
180 may be removed by, e.g., a wet etch. During the removal of the
mask 180, a small portion of the STI 160 may also be removed, but
not an amount sufficient to impact device performance. A second
mask (not shown) is selectively formed over the first insulator
layer 500, exposing the second portion 150 of the substrate. The
second mask may include silicon nitride formed by, e.g.,
deposition, photolithography, and a wet or a dry etch. A second
insulator layer 510, also referred to herein as "second crystalline
oxide layer," may formed over the second portion of the substrate
110 by a deposition technique such as CVD or ALD. After the
formation of the second insulator layer 510, the second mask may be
removed by, e.g., a wet etch. The first and second insulator layers
500, 510 may include the same materials and have the same
thicknesses as the crystalline oxide layer 400 described above with
reference to FIG. 9.
Referring to FIG. 15, a channel layer 520 may be formed over the
first and second insulator layers 500, 510, as well as over STI
region 160. A portion of the channel layer 520 disposed over the
STI region 160 may be subsequently removed by, e.g., CMP. In an
alternative embodiment, a first portion of the channel layer 520 is
formed over the first insulator layer 500 after the formation of
the first insulator layer and before the mask 180 is removed.
Similarly, a second portion of the channel layer 520 is formed over
the second insulator layer 510 after the formation of the second
insulator layer and before the second mask is removed. The channel
layer 520 may include a semiconductor material suitable for device
channel formation, such as a group IV material, e.g., silicon,
SiGe, germanium, or an array of carbon nanotubes; a III-V material
such as gallium arsenide, indium arsenide, indium gallium arsenide,
indium phosphide, gallium nitride, indium antimonide, gallium
antimonide, gallium phosphide; or a II-VI material, and mixtures or
alloys including one or more of the aforementioned materials.
In an embodiment, the channel layer 520 includes strained silicon.
The first insulator layer 500 may include a material having a
lattice constant larger than that of silicon, e.g., greater than
5.43 .ANG., resulting in the portion of the channel layer 520
disposed over the first insulator layer 500 to be tensilely
strained, and thereby suitable for use as the channel material of
an nMOSFET. The second insulator layer 510 may include a material
having a lattice constant smaller than that of silicon, e.g., less
than 5.43 .ANG., resulting in the portion of the channel layer 520
disposed over the second insulator layer 510 to be compressively
strained, and thereby suitable for use as the channel material of a
pMOSFET.
In an embodiment, a first portion of the channel layer 520 disposed
over the first substrate portion 140 differs from a second portion
of the channel layer 520 disposed over the second substrate portion
150. Different combinations of channel and crystalline oxide
materials may result in improved device performance in different
regions of the substrate.
In an embodiment, the use of different crystalline oxide materials
in first and second substrate portion 140, 150 facilitates the
deposition of channel layers thereover that have crystalline
orientations or in-plane rotations different from each other, as
discussed previously with reference to FIG. 15. The crystalline
orientation and/or rotation for channel layer 520 may be different
in first and second substrate portions 140, 150. Likewise, if
different channel materials are used in each substrate portion 140,
150, the different channel materials may have different
orientations and/or in-plane orientations. In an embodiment, the
nMOSFET channel layer in first substrate portion 140 has a surface
crystalline orientation defined by any of the {100} family of
crystallographic planes, and the pMOSFET channel layer in second
substrate portion 150 has a surface crystalline orientation defined
by any of the {110} family of crystallographic planes. In another
embodiment, the in-plane rotation of the nMOSFET channel material
in substrate portion 140 is such that the nMOSFET channel is
parallel to any of the <110> family of crystallographic
directions, and the in-plane rotation of the pMOSFET channel
material in substrate portion 150 is such that the pMOSFET channel
is parallel to any of the <100> family of crystallographic
directions.
This method of providing devices including channel materials of
crystalline orientations and/or rotations different from each other
may be superior to other methods in which regions of different
orientations are provided prior to device fabrication. This method
provides self-aligned channel materials of different crystalline
orientations and/or rotations, i.e., only in desired regions bound
by device isolation structures. Additionally, this method enables
the fabrication of device channel layers of virtually any
crystalline orientation or rotation, the choice of which is not
bound by and does not necessarily relate to a crystalline
orientation or rotation of an underlying substrate or of a handle
wafer from which the layers may have been bonded. This method may
also be used to provide channel layers of arbitrary crystalline
orientation and/or rotation disposed over insulator layers.
Subsequently, an nMOSFET may be formed, including a channel
disposed in a portion of the channel layer 520 disposed over the
first insulator layer 500, and a pMOSFET may be formed, including a
channel disposed in a portion of the channel layer 520 disposed
over the second insulator layer 510.
The bilayer-on-insulator constructions described above and
illustrated in, e.g., FIG. 6, may give favorable carrier transport
behavior for FinFET-type devices, e.g., omega FETs, tri-gate FETs,
etc. FinFETs typically have gates that wrap around a channel on at
least two sides of a vertically oriented channel, allowing greater
control of channel charge than in a single gate device. This
configuration also has the potential to translate to higher drive
current and lower stand-by leakage current. Devices related to the
FinFET, such as the wrap-around gate FET (gate on both sides of as
well as above the channel), allow even more channel charge control
and hence even more potential for improved drive current and
leakage current performance.
Referring to FIG. 16, in an embodiment, a FinFET may be defined as
follows. A bilayer 600 is formed by deposition of two semiconductor
layers over insulator layer 120, as described above with reference
to FIGS. 1-3 and 6-8. The bilayer includes first semiconductor
layer 610 disposed over the insulator layer 120 and second
semiconductor layer 620 disposed over the first semiconductor layer
610. Both the first and second semiconductor layers 610, 620 may be
biaxially strained. The first semiconductor layer 610 may be, e.g.,
biaxially tensilely strained, i.e., tensilely strained in a plane
parallel to a top surface of the first semiconductor layer 610 and
compressively strained in a plane perpendicular to the first
semiconductor layer top surface. The second semiconductor layer 620
may be, e.g., biaxially compressively strained, i.e., compressively
strained in a plane parallel to a top surface of the second
semiconductor layer 620 and tensilely strained in a plane
perpendicular to the second semiconductor layer top surface. In an
alternative embodiment, either of the first semiconductor and
second semiconductor layers 610, 620 is uniaxially strained.
Referring to FIGS. 17A and 17B as well as to FIG. 16, the first
semiconductor layer 610 may be thicker than the second
semiconductor layer 620 such that the sidewall of a fin 630 defined
from the bilayer 600 is primarily formed from first semiconductor
layer 610. For example, the first semiconductor layer 610 may have
a thickness t.sub.1 of 40-400 .ANG. and the second semiconductor
layer 620 may have a thickness t.sub.2 of 10-100 .ANG.. Thus, the
bilayer 600 may have a total thickness t.sub.5 of, e.g., 50-500
.ANG..
The bilayer 600 may be patterned to define a plurality of fins 630.
In particular, fins 630 may be defined by the formation of a
photolithographic mask (not shown) over the bilayer 600, followed
by anisotropic RIE of the bilayer 600. Fins 630 may have a width
w.sub.1 of, e.g., 50-300 .ANG., and a height h.sub.1 approximately
equal to a thickness of the bilayer, e.g., 50-500 .ANG.. The
photomask/RIE steps also define source mesa region 632 and drain
mesa region 634. Fins 630, source mesa region 632, and drain mesa
region 634 include portions of the bilayer 600 not removed by RIE.
The photolithographic mask is removed after the RIE of the bilayer
600.
Referring to FIG. 18, in another embodiment, fins may be formed by
an alternative fabrication method. A single strained semiconductor
layer is disposed over the insulator layer 120 and substrate 110.
For example, the starting material may include a
strained-semiconductor-on-insulator (SSOI) wafer, with first
semiconductor layer 610 disposed over the insulator layer 120. The
first semiconductor layer 610 may be bonded to the insulator layer
120. Alternatively, the insulator layer 120 may include a
crystalline oxide, and the first semiconductor layer 610 may be
deposited over the insulator layer. The first semiconductor layer
610 may be substantially unstrained, or tensilely or compressively
strained.
Referring to FIG. 19, the first semiconductor layer 610 is
patterned to define a plurality of fins 630. The fins 630 may be
defined by the formation of a photolithographic mask (not shown)
over the first semiconductor layer 610, followed by anisotropic RIE
of the first semiconductor layer 610. The fins 630 may have a width
w.sub.1 of, e.g., 50-300 .ANG., and an initial height approximately
equal to a thickness of the first semiconductor layer, e.g., 50-500
.ANG..
Referring to FIG. 20, a plurality of sidewall spacers 640 may be
formed proximate sidewalls of the fins 630 as follows. A thin
conformal insulator layer is deposited over the fins 630 and
exposed portions of the insulator layer 120 between the fins. The
conformal insulator layer may be, for example, silicon dioxide or
silicon nitride, formed by, e.g., CVD, and may have a thickness of,
e.g., 100-1000 .ANG.. The sidewall spacers 640 is defined by the
removal of portions of the conformal insulator layer by, e.g., an
anisotropic dry etch, such that the remaining portions of the
conformal insulator layer (which define spacers proximate the fin
sidewalls) remain.
Referring also to FIG. 21, after the formation of the sidewall
spacers 640, a top portion of the fins 630 includes exposed
portions of a top surface of the first semiconductor layer 610 from
which the fins 630 are formed. Second semiconductor layer 620 is
selectively deposited on the exposed portions of the first
semiconductor layer. The selective deposition may be performed by
CVD, and the semiconductor precursor gas (or gases) may be
accompanied by a Cl-containing species, e.g., Cl.sub.2 or HCl gas.
Alternatively, the semiconductor precursor gas may be a chlorinated
species, e.g., dichlorosilane (SiH.sub.2Cl.sub.2), trichlorosilane
(SiHCl.sub.3), silicon tetrachloride (SiCl.sub.4), or germanium
tetrachloride (GeCl.sub.4). The second semiconductor layer 620 may
have a thickness t.sub.2 of, e.g., 10-200 .ANG.. The spacers are
removed, e.g., by a wet etch. In an embodiment, the top portion of
the fins 630, i.e., the top portion of exposed first semiconductor
layer 610, are removed by an in-situ etch prior to deposition of
the second semiconductor layer 620. Such an etch may preserve the
aspect ratio of subsequently formed bilayer fins 630.
Referring to FIGS. 21 and 16, the resulting fins 630 include second
semiconductor layer 620 disposed over first semiconductor layer
610, like the fins formed from bilayer 600. In both embodiments,
the top surface of the fin may include a material different from
the material disposed along a major portion of the sidewalls of the
fin. In both embodiments, both the sidewalls of the fins and the
top surfaces may exhibit a same type of strain, i.e., compressive
or tensile.
In summary, this bilayer fin formation may be accomplished by
forming fins from an existing bilayer material. Alternatively, the
bilayer fins may be initially defined by a single material that is
then epitaxially capped with a second material. In both
embodiments, the first semiconductor layer is preferably thicker
than the second semiconductor layer.
Carrier transport in FinFET device constructions may occur along
three different planes of the device: over a top surface of a fin
and along first and second sides of the fin.
If a FinFET includes a single strained semiconductor layer disposed
over an insulator layer, atoms biaxially strained along one plane
will have an opposite type of strain in a perpendicular plane
(e.g., Si that is tensilely strained in a horizontal direction will
be compressively strained in the vertical direction due to Poisson
deformation of the lattice). Hence, conduction along a sidewall of
a fin covered by strained Si will be through a compressively
strained layer, and conduction along a top of the fin will be
through a tensilely strained layer.
In some embodiments, it may be preferable to have a construction
exhibiting primarily the same strain on all three sides of the fin,
i.e., both horizontally and vertically, in order to maximize the
mobility of carriers conducting parallel to all three sides of the
fin. Therefore, a tensilely strained film may be formed over the
insulator layer and topped with a compressively strained film.
Here, most of the sidewall conduction will be through the
compressively strained sidewalls, and conduction along the topside
of the fin will be through the other compressively strained
material disposed over the tensilely strained film.
Referring to FIG. 22, in another embodiment, the fins are at least
partially defined in crystalline oxide layer 400. The crystalline
oxide layer may have a thickness t.sub.7 of, e.g., 50-1000 .ANG..
First semiconductor layer 610, having a thickness t.sub.1 of, e.g.,
10-200 .ANG., is disposed over the insulator layer including
crystalline oxide. A photolithographic mask (not shown) may be
defined over the first semiconductor layer 610. The
photolithographic mask is, for example, photoresist. A hard mask
700, such as a silicon nitride hard mask, is defined over the first
semiconductor layer 610. This hard mask 700 may enable subsequent
formation of a semiconductor layer on fin sidewalls without also
forming the semiconductor layer on the top surfaces of the fin. The
hard mask may also act as a CMP stop (see below), and may have a
thickness t.sub.8 of, e.g., 100-1000 .ANG..
Referring to FIG. 23, fins 630 may be defined by anisotropic RIE of
the first semiconductor layer 610 and the crystalline oxide layer
400. Fins 630 may have a width w.sub.1 of, e.g., 50-300 .ANG., and
a height h.sub.1 approximately equal to a sum of the thickness
t.sub.1 of the first semiconductor layer and at least a portion of
the thickness t.sub.7 of the insulator layer 400, e.g., 50-500
.ANG..
Referring to FIG. 24, second semiconductor layer 620 may be
conformally deposited over and between the fins. The second
semiconductor layer 620 may be chosen to be deliberately
lattice-mismatched to the crystalline oxide 400 such that the layer
620 is under tensile or compressive strain, as appropriate. The
layer 620 may contain a semiconductor material identical to or
substantially the same as that included in the first semiconductor
layer 610.
Referring to FIG. 25, a thick oxide fill material 710, e.g.,
SiO.sub.2, may be deposited over and between the fins 630.
Referring to FIGS. 25 and 26, a planarization step, such as a CMP
step, may be performed to planarize the oxide fill material 710 and
to remove a top portion of the second semiconductor layer 620
disposed over the hard mask 700, thereby exposing the hard mask
disposed on the tops of the fins 630.
Referring to FIGS. 26 and 27, after planarization, the oxide fill
material 710 may be removed by a wet or a dry oxide etch.
Subsequently, an anisotropic dry etch may be used to remove
portions of the second semiconductor layer 620 disposed over the
hard mask 700 on the fins 630 and over the crystalline oxide layer
400 between the fins 630. The hard mask 700 can then be removed by
a wet or dry etch, exposing the first semiconductor layer 610
disposed on the tops of the fins 630.
The resulting structure has a semiconductor material disposed on
three sides of a crystalline oxide fin.
As discussed previously, crystalline oxide layer 400 may be
deliberately lattice-mismatched to semiconductor substrate 110 for
subsequent layer deposition. The crystalline oxide layer 400
material may be selected such that a major crystallographic planes
of crystalline oxide layer 400 parallel and/or perpendicular to a
surface of semiconductor substrate 110 is different from a
crystallographic plane of the surface of semiconductor substrate
110. This configuration may facilitate the desired lattice match or
mismatch between crystalline oxide layer 400 and semiconductor
substrate 110.
For example, semiconductor substrate 110 may include or consist of
silicon, SiGe, or germanium with a {100} surface, and crystalline
oxide layer 400 may include an aforementioned material with a
crystallographic plane other than {100}, e.g., {200}, {110}, or
{111}, parallel to the surface of semiconductor substrate 110. Such
a combination may provide an effective lattice constant of
crystalline oxide layer 400 suitable for lattice match or mismatch
with semiconductor substrate 110 or subsequently deposited layers.
This combination may also facilitate the formation of subsequently
deposited layers, e.g., first and second semiconductor layers 610,
620, having a crystallographic orientation (i.e., surface
crystallographic plane) different from that of semiconductor
substrate 110. Such changes in crystalline orientation may result
in enhanced carrier mobilities in devices subsequently fabricated
over fins 630.
This degree of freedom in selecting crystallographic orientation
may enable the formation of FinFETs having channel layers of
arbitrary crystalline orientation, e.g., having some FinFET
channels of different crystalline orientation with respect to
others, without necessitating complicated layout issues, device
rotations, or wafer bonding schemes. For example, in an embodiment,
a first FinFET including a crystalline oxide and a channel layer
having a substantially {100} crystalline orientation may be
fabricated adjacent to a second FinFET including a crystalline
oxide and a channel layer having a substantially {110} crystalline
orientation. Furthermore, the first and second FinFETs may be
fabricated such the devices (e.g., the fins) are oriented in
parallel to each other. Forming such a configuration may be
facilitated by the use of different crystalline oxides for each of
the first and second FinFETs. In this embodiment, the first FinFET
may be an n-channel device and the second FinFET may be a p-channel
device.
After the formation of the fins by any of the methods described
above, the completion of the FinFET fabrication may continue as
follows.
Referring to FIG. 28 as well as to FIGS. 17A and 17B, a gate
insulator layer 715 is formed over the fins 630 and exposed
underlying insulator layer 120 or crystalline oxide layer. Gate
insulator layer 715 is conformally formed over fins 630, as well as
over source and drain mesa regions 632, 634. Gate insulator layer
715 may include, e.g., SiO.sub.2, SiO.sub.x.sub.y, silicon nitride
(Si.sub.3N.sub.4 or other compositions), barium oxide (BaO),
strontium oxide (SrO), calcium oxide (CaO), tantalum oxide
(Ta.sub.2O.sub.5), titanium oxide (TiO.sub.2), zirconium oxide
(ZrO.sub.2), hafnium oxide (HfO.sub.2), aluminum oxide
(Al.sub.2O.sub.3), lanthanum oxide (La.sub.2O.sub.3), yttrium oxide
(Y.sub.2O.sub.3), yttrium aluminate, lathanum aluminate, lanthanum
silicate, yttrium silicate, hafnium silicate, zirconium silicate,
and doped or undoped alloys, mixtures or multilayers, thereof and
have a thickness t.sub.9 of, e.g., 10-100 .ANG.. In some
embodiments, gate insulator layer 715 is grown, and is therefore
formed only over exposed semiconductor surfaces, i.e., over top
surfaces of fins 630 and source and drain mesa regions 632, 634. In
other embodiments, gate insulator layer 715 is deposited, and is
therefore formed over an entire top surface of the fins and exposed
portions of the underlying insulator layer 120 or crystalline
oxide.
Referring to FIGS. 29A and 29B, a gate electrode material 720 is
conformally formed over gate insulator layer 715, including over
fins 630. Gate electrode material 720 may include a suitably
conductive material such as, for example, doped polysilicon, doped
polycrystalline SiGe, Al, Ag, Bi, Cd, Fe, Ga, Hf, In, Mn, Nb, Y,
Zr, Ni, Pt, Be, Ir, Te, Re, Rh, W, Mo, Co, Fe, Pd, Au, Ti, Cr, Cu,
and doped or undoped alloys, mixtures or multilayers thereof,
deposited by ALD or CVD, such as by UHVCVD, APCVD, LPCVD, or PECVD,
and have a thickness t.sub.10 selected from the range of, e.g.,
100-2000 .ANG.. A photolithographic mask (not shown) is formed over
gate electrode material 720. Portions of gate electrode material
720 are selectively removed by, e.g., RIE to define a gate 730
crossing over fins 630, and terminating in a gate contact area 740.
Portions of gate insulator layer 715 are exposed (or even removed)
by the RIE of gate electrode material 720.
Referring to FIGS. 30A and 30B, a plurality of dopants are
introduced into source and drain mesa regions 632, 634 to define a
source 750 and a drain 760. To form an n-type FinFET, dopants such
as arsenic, antimony, or phosphorus may be implanted into mesa
regions 632,634. Suitable implantation parameters may be, for
example, arsenic with a dose of 2.times.10.sup.15 atoms/cm.sup.2
implanted at an energy of 10-50 kilo-electron volts (keV). To form
a p-type FinFET, dopants such as boron or indium may be implanted
into mesa regions 632, 634. Suitable implantation parameters are,
for example, boron, with a dose of 2.times.10.sup.15 atoms/cm.sup.2
at an energy of 3-15 keV. For the formation of a CMOS device, NMOS
regions may be protected by a mask during the implantation of
p-type dopants into PMOS regions. Similarly, PMOS regions may be
protected by a mask during the implantation of n-type dopants into
NMOS regions. A suitable mask for both types of implantation may
be, e.g., photoresist.
During the introduction of dopants into source and drain mesa
regions 632, 634, a plurality of gate dopants 775 may also be
introduced into gate 730 and gate contact area 740. Gate dopants
770 serve to increase a conductivity of gate electrode material
720. Gate dopants 770 may be, for example, implanted arsenic,
antimony, or phosphorous ions for an n-type FinFET.
Dopants for both n-type and p-type FinFETs may be implanted at an
angle of 20-50.degree., with zero degrees being normal to the
substrate 110. Implanting at an angle may be desired in order to
implant ions into a side of exposed fins 630 and also into a side
of the vertical surfaces of gate electrode material 720.
Referring to FIGS. 31A and 31B, a blanket layer of spacer insulator
material is formed over the substrate 110, including over gate 730,
gate contact area 740, source 750, and drain 760. Spacer insulator
material may be, for example, SiO.sub.2 or Si.sub.3N.sub.4
deposited by CVD and have a thickness of, for example, 100-1000
.ANG.. Subsequently, portions of spacer insulator material are
removed by an anisotropic RIE to define a plurality of sidewall
spacers 775 proximate vertical surfaces, such as fins 630, gate
730, and gate contact area 740. Horizontal surfaces, such as top
surfaces of fins 630, are substantially free of the spacer
insulator material.
After the RIE definition of sidewall spacers 775, the portions of
gate insulator layer 715 exposed by the RIE of gate electrode
material 720 may be removed from top surfaces of source 750, and
drain 760 by, e.g., a dip in hydrofluoric acid (HF), such as for
5-30 seconds in a solution containing, e.g., 0.5-5% HF.
Alternately, this removal may be via RIE, with an etchant species
such as, e.g., CHF.sub.3.
Referring to FIGS. 32A and 32B, a salicide is selectively formed
over the substrate 110 to provide low-resistance contacts to the
source and drain regions and the gate electrode as follows. A
conductive layer is formed over the substrate 110. For example, a
metal such as titanium, platinum, zirconium, cobalt, nickel, or
alloys, mixtures, or multilayers thereof is deposited by, e.g., CVD
or sputtering, with the conductive layer having a thickness of,
e.g., 50-200 .ANG.. An anneal is performed to react the conductive
layer with the underlying semiconductor, e.g., exposed portions of
gate 730 and gate contact area 740, to form salicide 780 including,
e.g., cobalt silicide or nickel silicide. Anneal parameters may be,
for example, 400-800.degree. C. for 1-120 seconds. Unreacted
portions of the conductive layer disposed directly over insulator
material, such as exposed portions of insulator layer 120 and
sidewall spacers 775, are removed by a chemical strip. A suitable
chemical strip may be a solution including
H.sub.2SO.sub.4:H.sub.2O.sub.2 at a ratio of 3:1. A second anneal
may be performed to further lower resistivity of salicide 780. The
second anneal parameters may be, for example, 600-900.degree. C.
for 1-120 seconds. A FinFET 790 includes fins 630, gate insulator
715, source 750, drain 760, and gate 730, and an exemplary FinFET
790 having three fins 630 is illustrated in FIG. 32B. The three
fins 630 share a common source 750 and a common drain 760. A single
transistor may have multiple fins to increase current drive in
comparison to a transistor with a single fin. The semiconductor
material disposed in each fin 630 defines a device channel.
In an alternative embodiment, gate dielectric material may be
removed from the top surfaces of the source and drain mesa regions
immediately after the RIE of the gate electrode. In some
embodiments, raised source and drain regions may be formed, as
described above with reference to FIGS. 4 and 5.
Referring to FIG. 14C as well as to FIGS. 33A-33C, a structure
including first and second insulator layers 500, 510 disposed over
first and second portions 140, 150 of semiconductor substrate 110,
respectively, may be used to fabricate a first FinFET 790a and a
second FinFET 790b that are physically parallel to each other and
have channels with different crystalline orientation. Such first
and second FinFETs may be fabricated as follows.
First insulator layer 500 disposed over the first portion 140 of
semiconductor substrate 110 may include a first crystalline oxide,
and second insulator layer 510 disposed over the second portion 150
of semiconductor substrate 110 may include a second crystalline
oxide. The first and second crystallographic oxides may be
identical, substantially the same, or substantially different.
First semiconductor layer 610a comprising a first semiconductor
material is disposed over the first insulator layers 500, and first
semiconductor layer 610b comprising a second semiconductor material
is disposed over the second insulator layer 510. In an embodiment,
first and second semiconductor materials are identical or
substantially the same. In an alternative embodiment, first and
second semiconductor materials are different. In both embodiments,
the first semiconductor material disposed in first semiconductor
layer 610a has a first crystalline orientation, the second
semiconductor material disposed in first semiconductor layer 610b
has a second crystalline orientation, and the first and second
crystalline orientations are different.
A first plurality of fins 630a disposed over the first insulator
layer 500, and a second plurality of fins 630b disposed over the
second insulator layer 510 are defined as discussed above with
reference to FIG. 23-27, with a second semiconductor layer 620a
comprising the first semiconductor material disposed on the
vertically oriented sidewalls of the fins 630a, and a second
semiconductor layer 620b comprising the second semiconductor
material disposed on the vertically oriented sidewalls of fins
630b. At least one fin from the first plurality of fins 630a may be
parallel to at least one fin from the second plurality of fins
630b. The term "vertically oriented" does not denote a particular
absolute orientation; rather, it is used herein to mean an
orientation that is substantially perpendicular to a top surface of
the substrate over which the fin is formed.
In the embodiment in which the first and second semiconductor
materials are different, second portion 150 of substrate 110 may be
the portion of the substrate including second insulator layer 510,
and may be protected by a masking material during fabrication of
the first FinFET 790a. Likewise, after the first FinFET 790a is
fabricated, it may be protected by a masking material during
fabrication of the second FinFET 790b. Alternatively, the same
channel material, i.e., semiconductor material, may be deposited
over both first and second insulator layers 500, 510 as described
above with respect to FIG. 15, and the first and second FinFETs
790a, 790b may be fabricated in turn as described above with
reference to FIGS. 22-27.
The first FinFET 790a includes fins 630a, gate insulator 715,
source 750, drain 760, and gate 730. The second FinFET 790b
includes fins 630b, gate insulator 715, source 750, drain 760, and
gate 730. Both FinFETs 790a, 790b may include gate contact areas
740, sidewall spacers 775, and salicide regions 780.
The invention may be embodied in other specific forms without
departing from the spirit or essential characteristics thereof. The
foregoing embodiments are therefore to be considered in all
respects illustrative rather than limiting on the invention
described herein.
* * * * *